Automotive and pedestrian traffic is controlled at street intersections by traffic signals which function as visual cuing devices. While these devices vary with the perceived needs of a given intersection, essentially all provide red, green, and yellow signals to achieve an ordered use of an intersection. These basic signals may be implemented with turning arrows, pedestrian walking controls, and the like. When installed in metropolitan areas, each signal head of the traffic signal may be switched in networking fashion from a computerized traffic control system. Generally, such networking systems as well as in situ computer based controls provide switching outputs to replaceable solid-state load switches which are removably installed in controller boxes usually mounted at a given corner of an intersection. The load switches conventionally switch power on and off to a given lamp or lamps through the utilization of a gatable triac in conjunction with an opto-isolated gating component.
Traffic signals and their associated controls have been somewhat standardized by governmental regulation and industry standards. Such regulations and standards set forth physical parameters, electrical parameters, and those related to requisite lamp output or luminance so as to afford drivers and pedestrians consistent and safe visual cuing. In this regard, for example, NEMA Standard 5-18-1983 specifies the physical characteristics, general electric characteristics, and test procedures for a three circuit solid-state load switch. Thus, while improvements may be contemplated by investigators in the traffic control field, such improvements generally must accommodate these preset standards. In particular, any improvements contemplated may not compromise any of the safety standardized aspects of traffic control. For example, circuitry improving the efficiency of the cuing systems must not interfere with what otherwise is normal operation, i.e. it must be entirely "failsafe". In this regard, where add-on circuitry is employed, should it fail, then the normal traffic control input must not be compromised, the traffic signal performing in its accepted general fashion in the event of a drop-out of subsidiary components.
As may be expected, the cost of providing, as well as maintaining traffic control networks is substantial. A contemplation of the number of incandescent light bulbs, load switches, and the like for each intersection, and the number of intersections in a given metropolitan area demonstrates a very substantial governmental outlay to this one aspect of traffic control. In addition to the substantial costs of initial installation, maintenance considerations of the control network represent a substantial portion of the given governmental traffic control budget. For example, for the purposes of safety, the bulbs within every traffic signal of a given metropolitan area generally are changed on a regular preventive maintenance schedule, for example every six months. The procedure for bulb replacement itself is problematic in terms of the safety of working personnel. Personnel have been struck by moving vehicles during this procedure. Notwithstanding the safety hazards associated with this, the cost of electrical power for operating a traffic control network is quite substantial. To lessen end cost, some investigators have looked to the provision of small load regulators formed, for example, as socket insert carrying triacs, which drop the applied current and voltage to a given lamp to thus conserve power while still maintaining luminance within requisite standards. These approaches have failed, however, because the inserts position the bulbs out of their optimal focal point position with respect to an associated covering traffic signal lens. Additionally, the devices are in line and thus detract from the operational security of the control system. In particular, such devices have been shown to affect the bulb failure detection components of traffic control systems, shutting down the sequencing operation of associated traffic lights.